In the electrolytic industry, reduction of energy consumption is an important concern. Great efforts have been made, particularly, to reduce electrolytic voltage.
For example, in the electrolysis of a sodium chloride aqueous solution by an ion-exchange membrane method, improvements in factors causing an increase of voltage have been made as far as are technically possible to accomplish a reduction of voltage by, for example, adoption of an insoluble metallic anode having a coating containing a noble metal oxide, reduction of the electrode gap to a minimum, forced circulation of the electrolytic solution, or like technique. Similarly, various improvements with reference to reduction of overpotential of a cathode have been proposed. However, a cathode which is so durable as an anode and also retains a small magnitude of overpotential of several tens of millivolts over an extended period of time has not yet been developed.
Low-carbon steel, which had been used as a cathode material from the beginning of practical application of the ion-exchange membrane method, exhibits a relatively high hydrogen overpotential ranging from 300 to 400 mV. With the demand for the production of more highly concentrated sodium hydroxide, the low-carbon steel had been replaced with more anticorrosive stainless steel, nickel or a nickel-plated material. Reduction of hydrogen overpotential, however, could not be attained with these materials.
It was then found that the apparent over-potential can be decreased by 100 to 200 mV by increasing the surface area of the cathode by, for example, elution of Zn from an Ni-Zn alloy plating, plasma spray coating of Ni or Raney nickel, or suspension plating using a powderous material. However, the degree of reduction of voltage attained by these techniques is still insufficient. In addition, since the resulting cathode has a rough surface, it tends not only to damage an ion-exchange membrane but to accumulate iron ion, etc., in the electrolytic solution to have reduced activity which leads to a reduction of working life.
In recent years, cathodes comprising nickel as a main component combined with various catalytic components for the purpose of attaining a low over-potential have been widely used. For example, cathodes containing a copper or sulfur component as a catalytic component are known. However, since these components have insufficient durability, the cathode tends to deteriorate and fails to have a long life, though showing a reduced initial overpotential.
It is also known to use a platinum group metal or its oxide in a cathode so as to achieve a reduction of overpotential and extend durability. Examples of known cathodes using an oxide of a platinum group metal include one obtained by coating a heated metal base with a solution containing a salt of a platinum group metal followed by calcining to form a surface layer comprising a metal oxide, e.g., ruthenium oxide, as disclosed in Japanese Patent Publication No. 22556/80; one obtained by depositing a powderous oxide of ruthenium, etc., together with nickel onto a base surface by suspension plating, as disclosed in U.S. Pats. Nos. 4,465,580 and 4,238,311; one obtained by forming a composite oxide of nickel, etc., and ruthenium, etc., on the surface of a base as disclosed in Japanese Patent Application (OPI) No. 232284/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and the like. Although these cathodes have a low hydrogen overpotential and are hardly affected by impurities, such as iron in the electrolytic solution, the problem of durability still remains with the use of a labile metal oxide as a cathode, often resulting in short duration.
On the other hand, Japanese Patent Application (OPI) No. 23083/82 discloses a cathode comprising a base, e.g., nickel, on which a platinum group metal, particularly, platinum or an alloy thereof, is chemically deposited. This cathode exhibits a low hydrogen over-potential and durability but is still disadvantageous in that it is subject to deactivation due to impurities, such as iron in the electrolytic solution.
As described above, platinum group metals and oxides thereof are known to have a low hydrogen over-potential. In particular, metallic platinum exhibits excellent durability as a cathode. However, a cathode coated with platinum is so sensitive to impurities, particularly, iron ion in the electrolytic solution, that its activity would be lost due to even a trace amount, e.g., 1 ppm or less, of such impurities. In the actual electrolysis operation, materials of the electrolytic apparatus or pipes often contain iron, and it is very difficult to exclude iron ion from the electrolytic solution.